The present invention relates to a generator for pulsed electron beams. More particularly the present invention relates to an improved generator for pulsed electron beams which is particularly applicable for the tempering or annealing of solid surfaces.
In recent years, the short-time annealing or solid state surfaces, particularly semiconductors and metals, has become one of the important preparation methods in materials technology. This type of annealing is usually effected by light pulses of a duration of about 100 ns which are generated by lasers. This method of laser annealing (LA) is utilized inter alia to anneal radiation damage in implanted semiconductor crystals. The advantage of short-time processing in such implanted semiconductor crystals, compared to conventional baking in annealing furnaces, is mainly that, due to the short period of application of the light flash it is possible to completely anneal radiation damage but avoid a diffusion of the implanted foreign atoms. Thus in many semiconductors, the application of laser annealing (LA) has yielded foreign atom concentrations which are higher by orders of magnitude than it was possible with processing in the annealing furnace.
The properties of many semiconductor devices can be improved if short-time annealing is used during their manufacture. This applies, in particular, to solar cells. While defects or impurities may travel from the interior of the semiconductor crystal to the surface into the light-sensitive layer of the solar cell during long-term baking in a furnace, and thus considerably degrade the degree of efficiency of the cell, this is not the case with LA in which only the surface, i.e., the first several .mu.m of the crystal surface, is heated. Solar cells produced by means of LA therefore have an increased efficiency over the entire wavelength range.
The drawbacks of LA are mainly that the laser radiation is coupled into the semiconductor to be annealed in a greatly differing manner, namely in dependence on the type and concentration of the dopant. LA becomes particularly difficult when used with metals which often reflect up to 95% of the laser light.
Short-time annealing independent of conductivity and reflectivity can be realized by the use of pulsed electron beams. The thickness of the surface layer being heated up is closely correlated to the penetration depth of the electrons, which again depends directly on the impinging energy of the electrons. For annealing implanted surfaces of a thickness of several 100 nm, the impinging energy should lie in a range between 10 and 20 keV. Since the deposited pulse energy required for melting lies in the approximate range around 1 Joule/cm.sup.2, e.g., 1-3 Joule/cm.sup.2 for silicon, current densities of the electron beam of about 1000 to 3000 A/cm.sup.2 result with a 100 ns pulse duration.
A generator for pulsed electron beams is known, e.g. see IEEE Transactions on Nuclear Science, Vol. NS23, No. 5, October 1976, pages 1470-1477. However, this known generator for pulsed electron beams operates with a field emission plasma diode. The drawbacks of this prior art device are in particular that plasma diodes furnish homogenous and reproducible electron beams only if they are operated at voltages of at least several 100 kV. This is the case because in a plasma diode the electron emitting plasma layer is formed by gases which are generated by evaporation from many whiskers protruding from the cathode surface. These whiskers are vaporized by the Joule heat generated by their own field emission current. A large-area, reproducible whisker evaporation requires electrical field intensities of 200 kV/cm.
Another drawback of this prior art generator is that the operating voltage must be applied to the plasma diode within the very short time of 10 to 15 ns. This requires the use of gas discharge switches with very low inductance between the plasma diode and the energy store which comprises a coaxial capacitor filled with water as the dielectric material.